<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
	</body>
	<script type="text/javascript">
		var a = new Array( 3 );
		var b = [ undefined, undefined, undefined ];
		var c = [];
		c.length = 3;
		
		console.log(a.join( "-" )); // "--"
		console.log(b.join( "-" )); // "--"
		console.log(c.join( "-" )); // "--"
		a.map(function(v,i){ console.log(v,i)});//无输出 
		b.map(function(v,i){ console.log(v,i)});//有输出
		c.map(function(v,i){ console.log(v,i)});//无输出
	</script>
</html>
